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  1. #1
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    Aerodynamics when going down hill

    Two simple questions.
    When going down hill in a tuck, is it more aerodynamic to keep the cranks at 9 and 3 or 12 and 6 ?

    Question two.
    If going down hill in the tuck if one turns the cranks, even without applying significant power, does this motion churn the air and reduce the air drag? So, is turning the cranks better than maintaining a motionless position?

    Answers and opinions welcomed.

  2. #2
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    Quote Originally Posted by MGS9500 View Post
    Two simple questions.
    When going down hill in a tuck, is it more aerodynamic to keep the cranks at 9 and 3 or 12 and 6 ?

    Question two.
    If going down hill in the tuck if one turns the cranks, even without applying significant power, does this motion churn the air and reduce the air drag? So, is turning the cranks better than maintaining a motionless position?
    Question 1: 9 and 3
    Question 2: yes it churns the air and therefore it increases drag. No, it is not better.

  3. #3
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    #1
    If you looked at a front-on profile of the cyclist, the cross-sectional area in most cases at minimized when cranks are @ 9 & 3, and so the drag ought to be also be least. That's a bit of simplification, but seems to me that's what most pro cylists do, too.

    However, if there's a bend or turn in the road, you have more control when the outside leg & crank is at the bottom -- and I'd rather have full control of the bike!

    #2
    Pedaling cranks likely conflicts with the "9 & 3", but my legs start to cramp if I keep them in that fixed position for more than a few minutes at a time, so I do some pedaling even on steep downhills -- I want to be ready when the road turns up again.

  4. #4
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    Here are my thoughts:
    #1
    The cross sectional area of the rider is independent of crank angle. Think of it this way. With the cranks level, you have a given cross section. Rotate the cranks a bit and one leg extends and the other contracts, both by the same amount, so the cross section remains the same. The cross section area of the cranks themselves is greater at 12:00/6:00 than when level. A second effect is the aerodynamic shape of the rider. A leg extended is a roughly a cylinder perpendicular to the wind. With the leg bent, the thigh gets closer to horizontal and the shin is angled back slightly from the knee toward the foot, making the leg itself more aero. However, with the legs bent, the tendency is to have the toes angled downward which increased the drag of the foot. Soooo, I keep the cranks level and my the soles of my feet level. I'm perfectly content to corner fast with the cranks level and usually do. Perhaps this stems from many years of motorcycling where I had no choice? IMO, cornering with the outside foot down rather than level is primarily psychological rather than physical.

    #2
    At a cadence of 100 rpm, your foot is moving in a circle at ~ 4mph. IMO, this will have a small but IMO negligible effect on drag. For me going downhill, it seems somewhere around 35 mph is when carefully staying in a tight tuck trumps coming out of it a bit in order to pedal.
    ... 'cuz that's how I roll.

  5. #5
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    I have done a lot of very long straight descents with consistent grade. That makes it easy to see the effects of body position.

    Holding the pedals at 9 and 3 is more aero than 12 and 6.

    Spinning the pedals will slow you down quite a bit. Not because of the aero difference in your legs/feet, but because you have to come out of your aero tuck to do so. If you can pedal while tucked then your tuck could be improved.

    The tuck is more important than foot position.

  6. #6
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    Quote Originally Posted by looigi View Post
    #2
    At a cadence of 100 rpm, your foot is moving in a circle at ~ 4mph. IMO, this will have a small but IMO negligible effect on drag. For me going downhill, it seems somewhere around 35 mph is when carefully staying in a tight tuck trumps coming out of it a bit in order to pedal.
    I live and ride mostly around Washington DC where there are not too many long straight downhill roads. On these smaller "climbs" I can't descend faster than 35 mph UNLESS I pedal. However, I only weigh between 155 and 160 so YMMV.

  7. #7
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    Quote Originally Posted by ericm979 View Post
    I have done a lot of very long straight descents with consistent grade. That makes it easy to see the effects of body position.

    Holding the pedals at 9 and 3 is more aero than 12 and 6.

    Spinning the pedals will slow you down quite a bit. Not because of the aero difference in your legs/feet, but because you have to come out of your aero tuck to do so. If you can pedal while tucked then your tuck could be improved.

    The tuck is more important than foot position.
    That's a score of 2 correct out of a possible 2.

  8. #8
    merckxman
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    You can go faster by outside foot down (and weighting it). Here is a sequence of a very fast downhill corner:
    ITALIAN CYCLING JOURNAL: Descending Technique on Pramartino

    Quote Originally Posted by looigi View Post
    Here are my thoughts:
    IMO, cornering with the outside foot down rather than level is primarily psychological rather than physical.

  9. #9
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    Aerodynamics when going down hill

    Quote Originally Posted by looigi View Post
    Here are my thoughts:
    #1
    IMO, cornering with the outside foot down rather than level is primarily psychological rather than physical.
    This is true if you remain seated. If you're putting any pressure on the pedals then it has an effect on handling and is an improvement to have the outside foot down.

  10. #10
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    Quote Originally Posted by Buzzaro View Post
    This is true if you remain seated. If you're putting any pressure on the pedals then it has an effect on handling and is an improvement to have the outside foot down.
    How so? It's an often-repeated 'truth', with 'facts' like 'a lower center of gravity' - but that's nonsense.

    If someone wants to argue that it somehow firms up the body-bike connection, I'll listen. Or if someone holds that it induces moving the torso inward, thereby slightly reducing the lean angle of the tire patch, I'll accept the reasoning (though somewhat question the value.)

    I'm not saying it's wrong to do so, just that I've never heard anyone offer a solid explanation.
    A good habit is as hard to break as a bad one..

  11. #11
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    Aerodynamics when going down hill

    Quote Originally Posted by danl1 View Post
    How so? It's an often-repeated 'truth', with 'facts' like 'a lower center of gravity' - but that's nonsense.

    If someone wants to argue that it somehow firms up the body-bike connection, I'll listen. Or if someone holds that it induces moving the torso inward, thereby slightly reducing the lean angle of the tire patch, I'll accept the reasoning (though somewhat question the value.)

    I'm not saying it's wrong to do so, just that I've never heard anyone offer a solid explanation.
    It changes where the weight is placed on the bike and the direction that weight puts force on the tires contacting the ground. The bike will handle differently depending on whether the the inside, outside, both pedals, or the seat has the weight or how it's divided amongst them. With the outside pedal you've got the weight acting very low on the bike and essentially the force is on a lever arm pushing the tire to the ground.

  12. #12
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    Quote Originally Posted by Buzzaro View Post
    It changes where the weight is placed on the bike and the direction that weight puts force on the tires contacting the ground. The bike will handle differently depending on whether the the inside, outside, both pedals, or the seat has the weight or how it's divided amongst them. With the outside pedal you've got the weight acting very low on the bike and essentially the force is on a lever arm pushing the tire to the ground.
    Having race motorcycles and bicycles on and off road, and as an engineer listening to stuff like this put forth by non-technical people in all those endeavors over 40+ years , this is mostly specious bull, IMHO.

    If you're not falling over, the total acceleration (gravitational + centripetal) is directed in a line from the contact patch of the tires through the CG of you and the bike. Anything you do weighting or "leaning" your body or the bike doesn't change this. All it does is change the handling characteristics a bit with respect to over and under steer due to the effect of fork rake an trail. Lean the bike under you (or lean your body out) and the bike will tend towards over steer. Lean your body in or the bike out and it will tend toward under steer. This affects how it feels. It also can be exploited in situations where you may want to intentionally slide the rear wheel (most often on dirt track or off road) or keep it from sliding, but it depends on the details and dynamics of situation (brake vs power slides, traction, terrain) as well as other factors.
    Last edited by looigi; 05-27-2013 at 09:53 AM.
    ... 'cuz that's how I roll.

  13. #13
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    Re: Aerodynamics when going down hill

    Quote Originally Posted by looigi View Post
    Having race motorcycles and bicycles on and off road, and as an engineer listening to stuff like this put forth by non-technical people in all those endeavors over 40+ years , this is mostly specious bull, IMHO.

    If you're not falling over, the total acceleration (gravitational + centripetal) is directed in a line from the contact patch of the tires through the CG of you and the bike. Anything you do weighting or "leaning" your body or the bike doesn't change this. All it does is change the handling characteristics a bit with respect to over and under steer due to the effect of fork rake an trail. Lean the bike under you (or lean your body out) and the bike will tend towards over steer. Lean your body in or the bike out and it will tend toward under steer. This affects how it feels. It also can be exploited in situations where you may want to intentionally slide the rear wheel (most often on dirt track or off road) or keep it from sliding, but it depends on the details and dynamics of situation (brake vs power slides, traction, terrain) as well as other factors.
    Over the last couple of years on RBR, I have learned to never bring engineering rigor to an RBR thread because there are people that "that been racing for decades" or "own a bike shop" and they all know more than Newton, Euler, or D'Alambert.

  14. #14
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    Quote Originally Posted by bwbishop View Post
    Over the last couple of years on RBR, I have learned to never bring engineering rigor to an RBR thread because there are people that "that been racing for decades" or "own a bike shop" and they all know more than Newton, Euler, or D'Alambert.
    I ain't never heard of any of those guys, except maybe the last guy, but he spells it "Jalabert." Are they bike shop owners?





  15. #15
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    Quote Originally Posted by Buzzaro View Post
    It changes where the weight is placed on the bike and the direction that weight puts force on the tires contacting the ground. The bike will handle differently depending on whether the the inside, outside, both pedals, or the seat has the weight or how it's divided amongst them. With the outside pedal you've got the weight acting very low on the bike and essentially the force is on a lever arm pushing the tire to the ground.
    Please explain how the tires can tell whether you are seated or have one pedal up or the other. The only thing the tires "see" is the total force (centripetal plus gravitational) and the angle of the frame to the ground. If any of these antics of foot placement, etc. change the lean angle then sure. But you can change the lean angle by any number of changes in body position.

    Between Newton and Euclid, you have your work cut out for you to explain this.

  16. #16
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    Aerodynamics when going down hill

    Quote Originally Posted by Kerry Irons View Post
    Please explain how the tires can tell whether you are seated or have one pedal up or the other. The only thing the tires "see" is the total force (centripetal plus gravitational) and the angle of the frame to the ground. If any of these antics of foot placement, etc. change the lean angle then sure. But you can change the lean angle by any number of changes in body position.

    Between Newton and Euclid, you have your work cut out for you to explain this.
    I'm not sure if you're referring to just pedal position or how they are weighted. If its just position, then no it should have any effect other than aerodynamic. But as far as weighting pedals it does. If I'm going straight and I put pressure on a pedal I need to pull on the opposite bar to keep the bike upright and straight. Am I wrong that centripetal force on one pedal would be any different while turning? Maybe I'm thinking about it incorrectly. After all I still watch boats sail off the edge of the earth while at the beach and laugh at them.

  17. #17
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    In regards to the tangent on foot down stuff I thought it was obvious you have the outside foot down when turning so that your inside pedal is up and won't hit the ground when leaning into a turn...

  18. #18
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    Quote Originally Posted by gumbafish View Post
    In regards to the tangent on foot down stuff I thought it was obvious you have the outside foot down when turning so that your inside pedal is up and won't hit the ground when leaning into a turn...
    Now THIS is an argument that I can believe! Weight distribution is what it is, regardless of which pedal you put the weight on. But if you have the inside pedal down and you lean too far... you'll be butt skiing in short order. Don't ask how I know this; I just do.

    Watch the track motorcycle riders take corners. They lean inside of the bike in the turns. That way, the bike doesn't have to lean quite as far and can stay on the grippy part of the tire instead of the sidewalls.

  19. #19
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    If the gradient is something 6+%, that means you'll be hitting 35 mph easily, then you're better off tucking in, and hangon for the ride.

    Regarding cornering. Let's just keep it simple. The fastest descenders are those railing corners with the outside foot down. One does not need to understand physics to see this.

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